Efforts to reduce emissions and improve combustor performance in gas turbine engines have brought about the use of staged fuel systems wherein one or more fuel injectors are used for low speed (low power) engine operation and one or more other fuel injectors are used for higher speed (higher power) engine operation. The fuel injectors used for low speed engine operation are commonly referred to as pilot or primary fuel injectors for providing a continuous fuel flow to the engine combustor for all regimes of engine operation from idle to high speed operation. The fuel injectors used for higher speed engine operation are referred to as secondary or main injectors for providing supplemental fuel flow to the engine combustor in an on-demand manner when higher engine speed (power) is required. The secondary fuel injectors thus are rendered non-operative during idle operation of the engine when the supplemental fuel flow is not needed and are rendered operative during higher speed (power) engine operation when supplemental fuel flow is required.
Airblast fuel injectors have been used in non-staged gas turbine engine fuel systems and are described in the Helmrich U.S. Pat. No. 3,684,186 issued Aug. 16, 1972 and the Simmons U.S. Pat. No. 3,980,233 issued Sep. 14, 1976. Airblast fuel injectors are designed to achieve atomization of a film of liquid fuel formed on a fuel discharge orifice surface or lip by directing high velocity airflow supplied from the engine compressor to an air passage of the injector at the fuel film as it leaves the fuel discharge orifice surface.
Airblast fuel injectors have been proposed for use in a staged gas turbine engine fuel system. However, use of airblast fuel injectors in a staged fuel system may present a so-called coking problem when the airblast fuel injectors comprise the secondary (or main) injectors of the system that are rendered non-operative during some engine operational regimes (e.g., low power regime such as low speed idle operation) and operative during other engine operational regimes (e.g., high speed operation). In particular, when the secondary (or main) fuel injectors are rendered non-operative during low power engine operation, fuel remains in the fuel discharge passages of the injectors and can be heated by the elevated temperature of the combustor environment to the extent that the fuel in the fuel discharge passages is carbonized (coked) over time, building up carbonaceous deposits therein. These deposits can adversely affect the performance of the airblast fuel injector in service from the standpoint that injector fuel flow rate and fuel atomization become unsatisfactory. Similar deposits may build-up in the fuel passages of other air atomizing fuel injectors that employ high velocity compressor airflow to atomize fuel discharged form a fuel discharge passage/lip.